The Study of Optoelectronic Properties and Functional Devices of Graphene

Author:Wang Wen Hui

Supervisor:yang yong hong ni zhen hua


Degree Year:2017





Graphene,a single layer of carbon atoms arranged in hexagonal honeycomb,has aroused significant interests in electronic and optoelectronic applications due to its unique zero-band structure,ultra-high carrier mobility and broadband absorption.Recently,graphene-based detectors,sensors and other high-performance devices are constantly studied and reported.However,the low light absorptivity of graphene limits the application of graphene in photodetectors to a certain extent.Based on this,many methods are employed to increase the light absorption or photo-excited carrier separation rate for improving the performance of photodetectors,such as adopting asymmetric electrode,PN junctions,optical microcavity,waveguide integration,as well as plasmonic structure.In addition,quantum dots,heterostructures are adopted to introduce a high gain mechanism in photodetector,which have greatly improved the performance of the detector.Nonetheless,graphene photodetectors still need to be further improved in the cost,preparation technology,performance stability.In order to promote the application of graphene,another method is to develop new functional devices,expanding new applications.Currently graphene-based functional devices mainly include gas sensors,pressure sensors,biological detectors,PH sensors,heavy metal detectors and so on.The emergence of those new detectors,not only accelerate the development and application of graphene,but also provide an opportunity for the development of the corresponding detector.Here,the modulation method of optoelectronic properties of graphene have been studied,and different functional detectors have been fabricated and studied.The specific contents are as follows:1.A junction in graphene device was prepared by using laser irradiation.The Raman spectrum demonstrates that P-type doping is introduced in the radiation region,and the doping degree can be modulated by controlling the irradiation time.The doping degree dependence of photocurrent generated at the P-P + junction shows that the photocurrent increases with P-doping degree.Then,the mechanism of photocurrent generation at the junction is analyzed carefully and the photothermal effect is proved to dominate the generation of photocurrent at the junction.2.A position-sensitive-detector based on graphene-SiO2/Si structure was designed and fabricated.The detector eliminates the PN junction or Schottky junction structure in general position-sensitive-detectors,and smartly utilizes the band bending caused by the SiO2/Si interface state and the lateral diffuse of the carrier at the SiO2/Si interface,achieving the lateral photoeffect.The electrons diffuse to the region under graphene will lead to an effective gating effect,consequently changing the channel current through capacitive coupling.The quantity of the carriers under the graphene channel varies with the incident light position,suggesting that different photocurrent will take place,so that the position of the incident light can be detected.Signal amplification in the order of~104 has been demonstrated due to the high mobility of graphene,which improve the detection limit of Si-based PSDs from μW to nW level,without sacrificing the spatial resolution and response speed.Such interfacial amplification mechanism is compatible with current Si technology and can be easily extended to other sensing systems.3.A passive position-sensitive-detector based on graphene-Si structure was presented.The graphene is used as a photon absorbing and carriers separation and diffusion layer.The high mobility allows the carriers to diffuse far away,ensuring large active area of the device.The quantity of carriers which arrived at the electrodes depends on the incident light position,suggesting that the position can be detected by measuring the voltage output of the electrodes.Position sensitive characteristics indicate that the detection of long-range(>8 mm)weak light at nWs level can be achieved.More importantly,it shows very fast response and low degree of non-linearity of-3%,and extends the operating wavelength to the near infrared(IR)region(1319 nm and 1550 nm).4.An electrostatic detector based on graphene was prepared,which realized fast and highly sensitive electrostatic detection.The detector takes advantage of the SiO2/Si interface gating effect and the high mobility of graphene.The movement of carriers in Si induced by electrostatic field is reflected by the current response of graphene with a high gain.The detector can sense~5 V of the electrostatic potential,and the response time is less than 2 μs.The electrostatic response modulation can be achieved by electrical regulation.Moreover,the non-contact electrostatic detector features simplicity in fabricate process,excellent in performance and low cost,which will have a good application in portable and flexible sensing fields.